Fix concurrent LRFinder instances overwriting each other by using tempfile

[warner-benjamin]

Resolves #3415 and resolves #3240 by saving model to TemporaryDirectory under learner path/model_dir.

Tested on Linux and Windows.

If preferred, I can update the PR to use TemporaryDirectory as a context manager in lr_find and pass the temp_dir name to LRFinder.

Notes:

path = self.path/self.model_dir
path.mkdir(parents=True, exist_ok=True)

is required as path/model_dir might not exist yet. And

self.tmp_p = Path(self.tmp_d.name).stem

exists because self.tmp_d.name returns model_dir/tempfolder.

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[jph00]

Many thanks!